This paper proposes a nonlinear magnetic method to estimate wall thinning of carbon steel pipe used in the secondary steam cycle of nuclear power plants. The principle of the proposed method is based on utilizing the nonlinear property of magnetic saturation in the carbon steel. A magnetic sensor system for the nonlinear magnetic method is designed by the magnetic circuit concept, and is modeled and analyzed with the finite element method and an equivalent electric circuit. The experimental setup of the sensor system is built and tested for its performance of estimation. The data measured from the sensor system, that is used to estimate the wall thinning, are compared with the analysis results to demonstrate feasibility and validity of the proposed nonlinear magnetic method.